INVESTIGATION OF MOLECULAR ORDER AND DYNAMICS IN LIQUID-CRYSTALS CONFINED IN POROUS-MEDIA USING THE DIPOLAR-CORRELATION EFFECT ON THE STIMULATED ECHO

A new application of the stimulated echo pulse sequence is presented t hat permits the elucidation of molecular order and dynamics in a time scale between about 100 mu s and the spin-lattice relaxation time, The technique exploits the influence of dipolar coupling on the quotient of the amplitudes of the stimulated and primary echoes produced by the standard three 90 degrees-pulse sequence, Results obtained for a nema tic liquid crystal in bulk and confined in porous glass (mean pore dia meter 4 nm) are compared, In both cases the echo amplitude quotient os cillates as a function of the pulse spacing. In a bulk nematic crystal these oscillations originate from strong unaveraged dipolar interacti ons and directly reflect the molecular order in the material, In porou s glass a real nematic order is absent. In this case, the oscillations can be attributed to spin exchange between inequivalent protons. Exch ange rates are estimated. Copyright (C) 1996 Elsevier Science Inc.